If you don't have an aversion to doing a little bit of research on the motherboards offered, a decent starting point to (at the very least) obtain baseline pricing information would be
www.pricewatch.com. At/around the $200 price range they have some of the following:
ECS 755A-2 Retail Box MBoard w/ Cbls Man CD Audio LAN DDR400 ATA133 AGP 8x 5PCI ATX with CPU AMD Athlon 64 3000 DDR PC 3200 Soc 754 and heatsink fan - RETAIL BOX W/ CABLES MANUAL & CD - $193
Chaintech K8T800 MBoard with cpu - Socket 754 - 8xAGP - Serial ATA - DDR400 - 6 Chan Audio Lan with CPU AMD Athlon 64 3000 DDR Soc 754 and heatsink fan - $193
ECS 755A-2 Retail Box mboard - AGP 8X - DDR - 6 Chan Aud - Serial ATA - USB 2.0 - 5 PCI - 10/100 Lan with cpu - AMD Athlon 64 3000 - Includes Fan / Heatsink - Complete Combo Kit - $193
AGP is being phased out in favor of PCI Xpress, but if you're not a die-hard PC gamer you don't really need to shell out the ~$500 to acquire a "high end" PCI Xpress video card. Regardless, you should still be fine with a decent 8x AGP card (unless you're planning on investing a significant amount of time into Doom 3). Pricewatch.com has all kinds of motherboard/CPU combos, depending wholly on what parts you're wanting and the price range you're working with. It's really rather difficult to recommend which parts to purchase without knowing what you're planning to use the machine for (gaming, etc.).
As far as what difference the upgraded hardware would make, I have a 2.8 GHz system, a 1.6 GHz system, and a handful of lower-end systems (P3 systems ranging from 533MHz to 866MHz), and the only time (aside from playing "high end" games, of course) that I really recognize a noticeable performance increase is in startup/shutdown times. Other than that, the difference I've been able to notice was negligible (at best). Of course, your mileage may vary.
Hope this helps.